Important binders used in the production of these press materials made of organic (e.g. lignocelluloses-containing) or inorganic (e.g. glass-wool-containing) materials are binders based on polyisocyanates of the diphenylmethane group (PMDI). These materials have inter alia significant involvement as wood fiber insulation board in the construction sector.
PMDI-bonded wood fiber insulation board (also termed “WFIB” below) achieves thermal conductivity values around 40 mW/(m*K). It therefore provides insulation comparable with that provided by the insulation materials glass wool and mineral wool.
The method usually used in the prior art to produce rigid WFIB wets wood fibers with PMDI binders, compacts the mixture, and hardens same by exposure to steam. This process is described by way of example in DE 10242770 A1. Flexible wood fiber insulation board can be produced by using what are known as bicomponent fibers. By way of example, WFIB is produced by the process described in EP 2315652 A in densities of about 50 kg/m3 to 100 kg/m3 with a very soft fiber-mat-like structure. However, these fibers cannot be used to produce a rigid insulation board—as produced with PMDI binders.
PMDI binders currently used for WFIB are straight PMDI, prepolymers, and fresh mixtures based on PMDI and polyol.
EP 1 330 481 A discloses binder mixtures based on polyisocyanate prepolymers made of PMDI and of polyoxyalkyleneols having at least 15% (preferably from 50 to 100%) content of oxyethylene structural units, and states that these have the advantage that the wood fibers used can have hither residual moisture content, and that nevertheless products with good dimensional stability are still obtained.
DE 101 41 209 A1 discloses binders based on prepolymers comprising urethane groups, obtainable via reaction of polyisocyanates of the diphenylmethane diisocyanate group with hydroxy-functional polyethers having more than 60% by weight ethylene oxide (EC)) content, based on the total quantity of alkylene oxides used for the production of the polyethers. A considerable improvement in cold adhesive bond strength during processing is stated as advantage.
DE 3 328 662 A1 discloses a process for the production of press materials, preferably particle board, via hot pressing of raw materials mixed with polyisocyanates as binder with concomitant use of catalysts which can be activated by heat and which are based on ammonium salts. It is said that these can by way of example be dissolved in aqueous polyisocyanate emulsions of the type obtained with addition of emulsifiers such as polyvinylpyrrolidone. Addition of the catalysts, which are latent at room temperature, can control the production process.